A biopsy sample smaller than a pinhead was placed on a sterile towel at the Mayo Clinic in Rochester last year and whisked to a pathology lab. On arrival, the dab of human tissue was nowhere to be found.

“We were never able to discern what exactly happened to that specimen,” said Dr. Tim Morgenthaler, Mayo’s chief patient safety officer.

Nor could they test the sample to solve the mystery of their patient’s illness.

The misplacing of irreplaceable biological specimens has proved to be a vexing problem for Minnesota hospitals, which reported 31 instances in the 12 months that ended last Oct. 6.

The state’s 13th annual report on hospital “adverse events,” released Thursday, included 336 reportable mistakes, including operations on the wrong body parts and disabling medication errors. But few were as common as lost tissue samples, which also can have serious consequences.

“That small little tiny one-millimeter polyp belongs to a patient,” said Dr. Rahul Koranne, chief medical officer of the Minnesota Hospital Association, which released the report with the Minnesota Department of Health. “And that could have been a critical data point that could have meant the diagnosis of a cancer vs. a diagnosis of health.”

129: Pressure ulcers

72: Falls

31: Lost specimans

30: Wrong-site surgery

The report included four adverse events that resulted in deaths — three from patient falls and one from a medication error — and 106 that led to severe injuries. As in most years, the most common adverse events were pressure ulcers, often called bed sores, and patient falls. Four injuries were reported last year because patients had not received follow-up instructions or test results.

Since 2005, Minnesota hospitals have publicly reported on more than two dozen errors dubbed “never events,” because they theoretically were preventable with proper safety precautions.

Although embarrassing to hospitals, the report is intended to help them learn from mistakes and share lessons. For example, state officials noted that the number of fall-related deaths was the lowest since 2011 and reflected campaigns by hospitals to assess patients for risk and provide them with safeguards, such as lower beds and protective floor mats.

“Hospitals have been working super hard on identifying patients at risk for injury — not just saying, ‘Well, this patient is at high risk for falling,’ but, ‘What’s their risk for injury if they do fall?’ ” said Rachel Jokela, director of the Health Department’s adverse event reporting.

Many hospitals now assume that patients who fall without nurses or staff present have hit their heads, she said, and treat them to prevent the chance of severe brain injuries forming.

In contrast to falls, misplaced specimens have been reported only since 2014. The total number of incidents probably is low as a result, because many hospitals don’t report on new error categories right away, Jokela said.

Still, the problem has captured the attention of the state’s hospitals. Last year Mayo switched to a standard container for specimens being sent to labs. Essentia Health recently added a computerized lab system, which uses bar-coding and other measures to track samples, to its northern Minnesota hospitals.

Allina, whose hospital nurses went on strike twice last year, also reported a medication error at Abbott Northwestern Hospital in Minneapolis that occurred in September, during one of the walkouts. The error involved a replacement nurse infusing an excessive amount of epinephrine into the bloodstream of an asthmatic patient.

Allina had reported only 31 errors a year earlier. Jokela said last year’s increase did not appear to be tied to the strikes at five Allina Hospitals, because the errors were spread throughout the year. “It’s nothing that I can attribute to the staffing change or the replacement nurses,” she said.

Different kinds of errors stem often from the same root causes, such as poor communication or handoffs between hospital departments, said Dr. Rajesh Prabhu, Essentia’s chief patient quality and safety officer. “We have to make sure that the system is designed to take into account that humans are part of that process. I mean, errors will occur. We have to make sure we have built-in redundancies in the system to catch those cases.”

One error involved unsafe administration of a blood product that harmed a patient at Essentia’s St. Mary’s hospital in Detroit Lakes. Solutions included creating a checklist to make sure blood products are administered properly, and moving staff members to quieter and less distracting locations when filling out orders for blood products, Prabhu said.

Large hospitals reported the most adverse events. Mayo reported 38, while the University of Minnesota Medical Center, Fairview, reported 37. HealthPartners reported a combined 25 errors at Regions Hospital in St. Paul and Methodist Hospital in St. Louis Park.

The lost-specimen incidents reflected the increasing complexity of hospital procedures. In some cases, a doctor lost a polyp from a colon. Other times a sample was sent to the wrong hospital department or was lost in transport to an external lab.

But the risk of errors often appeared greatest in hectic operating room or emergency situations, Jokela said. Some tissue samples were simply set aside and nobody took responsibility for sending them to labs.

“ ‘I thought you sent it to the lab, and you thought I sent it to the lab — it’s that kind of thing,” Jokela said. “Hospitals are putting processes in place to make sure all those specimen orders are handled before the patient ever leaves the OR.”